1. Field of the Invention
This invention relates to signal transmission ranging systems such as radar, sonar, positioning systems and the like, and more specifically a method of reducing system errors caused by reception of secondary path signals.
2. Description of Related Art
Radio ranging systems use the time of propagation of radio signals to obtain the range (distance) between points of interest. A classic example of a radio ranging system is radar. With radar the observed time of the transmitted radar signal in propagating to the target and returning to the radar receiver, then scaled by the speed of propagation, accomplishes ranging. Sonar is a direct acoustic analog of radar and is included as a ranging system also amenable to application of the subject invention. With Global Positioning System, GPS, or GPS-like satellite positioning systems, the time at which a satellite signal is received is compared to the time the satellite transmitted that signal, information available to the user, and then scaled by propagation speed to obtain range from satellite to receiver. In satellite positioning systems, a user""s position is the information generally desired and is obtained by exploiting understood trilateration techniques using the inferred ranges to a constellation of satellites whose positions in space are a priori known.
In radio and acoustic ranging systems a commonly observed effect occurs due to the reception of one or more secondary path signals associated with reflecting surfaces positioned to provide those signals. In television, multipath, the term used to denote this phenomenon is manifested as undesired echoes or ghosts of the image transmitted. In radio ranging systems multipath is manifested as errors in range which cannot be directly compensated unless the parameters of the multipath signal are known. Although, the effect of multipath signals is generally deleterious in radar and sonar, the information on the multipath signal has been used to assist in defining target position. To illustrate the severity of multipath effects in an exemplary radio ranging system such as GPS, the ranging error incurred due to multipath, with and without the use of a current state-of-the-art technique to reduce those effects, is shown in FIG. 1.
Multipath interference is coherent, or nearly so, with respect to the direct path signal, i.e., it has the same or very nearly the same spectrum as the direct path signal. For that reason it is particularly difficult to mitigate its effects. Filtering and other such commonly used means in the art of suppressing undesired signals are ineffective with multipath.
A number of methods have been put to use with modest success to mitigate multipath effects in radar and GPS. The simplest is the use of a signal receiving antenna that substantially reduces the response of the receiving apparatus to wavefronts originating from the presumed direction of the reflected signal. However, when the direct path and the reflected secondary path signals arrive at the antenna from the same or nearly the same direction, an undesired degradation of the receiving apparatus response to the direct path signal is experienced. This can seriously compromise ranging performance.
More significant for mitigating multipath effects are methods broadly described as algorithmic. One of the known multipath mitigation algorithms now in use has a performance shown in FIG. 1. As described in the literature, the technique employs methods for decomposing the signal correlation function to permit inferring the multipath parameters: wavefront intensity, path separation delay, and path phase shift, of the secondary path wavefront relative to the direct path wavefront, and thereby backing out or removing its effects on the measurement of the direct path delay. This and other similar algorithms are now implemented in receivers which incorporate microprocessors or special purpose computing apparatus to execute code embodying these algorithms.
The method or process of the present invention for mitigating multipath in GPS or radar or sonar receivers, or other similar communication systems, implements a substantially different method of operation than the prior art as described above and provides an improved level of performance, in fact, near the limiting performance bounds with a substantially reduced computational burden. This is a crucial result because it makes feasible in practice accuracies of position fixes not heretofore achievable, thereby allowing numerous applications in the fields of radar, sonar and GPS or GPS-like satellite positioning systems not otherwise feasible. In sum, the accuracy achieved with the subject invention exceeds the accuracy achieved by the current state of the art by a substantial factor. The computational efficiency of the invention permits achieving this level of performance with a parsimony of computational apparatus not achieved in the present art.
One object of the invention is to reduce the calculation time in a method of finding maximum likelihood estimates of range in signal transmission and ranging systems in the presence of multipath signals.
Another object is to provide a two-dimensional search process practical in equipment with limited computation capability.
These objects are achieved by reducing the number of signal samples required in forming signal/modulation cross-correlations and by reducing the multiplication rate. The computational burden of the processor is thereby reduced with increased efficiency in determining the estimation statistic.
A further object is to provide an improved measure of range in the presence of multipath signals with minimal compromise of performance.
The direct propagation path delay of a received signal in the presence of corrupting secondary propagation path signals is optimally estimated according to a process that presents a direct path signal delay estimate and a secondary path signal additional delay estimate as a starting point in a two-dimensional plane. The received signals is cross-correlated with the modulation waveform at the direct path delay estimate and the direct path plus secondary path additional delay estimate. The modulation auto-correlation values for a signal at zero delay and at the secondary path additional delay are determined. These values are used in a set of four linear equations to determine the variables a, b, c and d. The values of a, b, c and d are used to compute the estimation statistic for the pre-selected starting point. The same process is again utilized to determine the estimation statistics for points immediately surrounding the pre-selected starting point. If the estimation statistic for each of the surrounding points is not larger than the estimation statistic at the pre-selected starting point, the delay coordinates of the pre-selected starting point estimate are the optimal estimates of direct path and secondary path additional delays. If the estimation statistic of a surrounding point is larger than the estimation statistic of the pre-selected starting point, the above process is repeated for the surrounding points.